It depends what medium I am working with.
If it is a painting, then I pretty much have to have something in mind. It actually happens that way anyway. I see what I want to paint first. When drawing it is a little of both, or sometimes I just start to draw.
When I work with polymer clay, some times I know what I want to make and other times, most times I just start to warm up the clay and play around and something comes out of it.
With beads, both larger beads and seed beads, again it can go both ways, but with beads it is more a make it up as I go along sort of thing. I may have picked a color or two that is not working out so have to change it.
With the peyote stitch I let the geometric-symmetric patterns evolve on their own.
When I sew, I know what I am making.
Overall- I would say I envision the thing I am going to make. Sometimes it comes out the same way, sometimes it doesn't.